As a newb to Fediverse, I agree because it is ambiguous how to use one account for several instances. I’ve browsed the web for several hours. But I only found out that the above is not a one-size-fits-all because some instances require registration.
Also, saying that an account can be created to access communities in my experience, implies I can only see and minimally interact on those instances. But I cannot go as far as posting anything because as I previously stated, I need an account on the said instance to do that.
I see the Fediverse being an umbrella of apps/services. However, from my experience, they’re not synchronized. More like silos.